1. Field of the Invention
The present invention relates to a method and system for measuring the time difference between a plurality of high-precision clocks. More particularly, the present invention relates to a simplified extended dual mixer time difference measurement system which employs a common oscillator as opposed to a synthesizer, thereby reducing the cost of the system and eliminating noise produced by a synthesizer.
2. Description of the Related Art
The ability to measure a precise period of time or keep accurate time has become increasingly important to both the scientific and commercial world in this era of high-speed computers and communications. The introduction of molecular or atomic clocks over 40 years ago brought timekeeping to an entirely new level of accuracy. Molecular clocks employ a molecular material, such as cesium or rubidium, which has a frequency output of a value which is essentially determined by the inherent characteristics of the material.
However, it was found that two molecular clocks employing the same material usually had frequency outputs that varied somewhat due to one or more of many factors. Factors which affect output frequency of molecular clocks include environmental factors such as temperature, the existence of magnetic fields, random fluctuations, frequency drift, and frequency and time offsets.
In the United States, the official "time" has been calculated by the National Bureau of Standards (NBS) from an ensemble of continuously operating cesium clocks. Frequency differences between clocks is addressed, with data of frequency calibrations and interclock comparisons being statistically processed to provide near-optimum time stability and frequency accuracy. The NBS time standard, as well as other similar standards, has been made available globally via satellites. Thus, the "time" has become available to parties at remote locations having the appropriate hardware and means for processing the signals. These signals, alone or in combination, were and remain used for several applications. For example, navigation systems of ships at sea utilize time signals from three or more of such satellites to determine their location. However, these applications require specialized receiving equipment, and are subject to problems from a number of sources, such as atmospheric interference, etc. Therefor, applications which require extremely high precision, reliability and/or some sort of detection avoidance are not best served by the satellite time signals.
Recently, the need for high-precision timekeeping has been ever increasing, and the use of dedicated molecular clocks has become quite common in a wide variety of applications. For example, naval vessels use molecular clocks to keep highly accurate time for a variety of functions, including classified communications and on-board tactical systems. Scientific experiments that are time dependent, especially in areas such as physics, often require that extremely accurate time measurements be made. Further applications include electronic monitoring or eavesdropping. However, in many of these applications, the use of a single molecular clock does not guarantee the precision timekeeping required. In many of these situations, the use of an ensemble of two or more clocks would provide the desired precision. But when an ensemble of clocks is used, the "time" is realized by processing the times and frequencies of the clocks together. However, the cost of the required signal processing hardware has been prohibitive and the reliability somewhat less than satisfactory, thereby limiting the use of ensembles in the face of an ever-increasing demand for precision that only an ensemble can provide.
As discussed above, when an ensemble of clocks is employed, hardware is necessary for comparing times and frequencies and deriving the differences so that a calculation of the "time" based on the output of all the clocks can be made. A variety of techniques for doing so are presently employed. One of the more advanced techniques is the extended dual mixer time difference measurement technique, which was developed by the present inventor. Like most prior art measurement techniques, the extended dual mixer technique ties the "time" from each clock to a time base, which is a signal having a known frequency synthesized from one of the clocks in the ensemble. One significant feature of the extended dual mixer technique is the use of scalers to count zero upcrossings in the beat signal derived from each clock. Prior dual mixer techniques were able to detect phase differences between beat signals, but an ambiguity problem remained that these techniques could not measure. This ambiguity is also referred to as a difference in the epoch of the signals. Over a period of time, frequency differences between the beat signals derived from different clocks often result in a difference in the number of cycles completed by respective beat signals. Generally, no error would be introduced over short measurement periods, as the epoch of the signals would ordinarily remain the same. However, over longer measurement periods, the total number of cycles completed would often be different for each beat signal, an error that the prior techniques did not address.
The extended dual mixer technique eliminated the ambiguity problem by adding scalers to count the zero upcrossing of each cycle of the beat signal for each clock. In this way, both the phase difference and the cycle ambiguity between clocks in an ensemble could be ascertained. This time measurement system required less supervision than its predecessors and provided data to a computer which permitted a more accurate representation of the time to be derived from the ensemble.
However, the extended dual mixer technique remains subject to problems that have haunted time measurement systems for ensembles. The most important of these problems are reliability, noise produced by the various components of the system, sensitivity of the components to environmental factors such as temperature, complexity and expense. Clearly, if the expense of such measurement systems was reduced and their reliability increased, clock ensembles and their required hardware would receive wider acceptance in the existing markets that are demanding precision timekeeping.